There is disclosed an improved pump (5, 5) particularly suitable for use in a method of "artificial lift" in an oil/gas well. Known pumps used in artificial lift methods suffer from a number of problems/disadvantages-e.g. low efficiency (hydraulic efficiency). The disclosed embodiments of the invention provide a pump (5, 5) which provides a positive displacement of a predetermined volume of well production fluid for each operative cycle of the pump-in contra-distinction known to pumps which provide axial flow of well production fluid. The inventive pump (5, 5) provides a chamber (10, 10) having a volume (V, V), an inlet (15) to the chamber (10, 10), an outlet (20) from the chamber (10, 10), and means for varying the volume (V, V) of the chamber (10, 10). The means for varying the volume (V, V) of the chamber (10, 10) is controlled by relative rotation of first and second bodies (30, 35, 30, 35).